1. Field of the Invention
The present invention relates to an apparatus for driving a stepping (stepper) motor, particularly incorporated in a digital still camera.
2. Description of the Related Art
Recently cameras have been employing a motor for driving the autofocus system. In the case of a DC motor, a relative position detecting means which detects a relative position with respect to a reference position using an encoder or the like is used as a means for detecting a position of a focusing lens. It is necessary to provide a space in which the encoder is accommodated in the camera. Also, a means for detecting the output of the encoder must be incorporated in the camera.
To this end, it is possible to use a stepping motor (stepper motor) in which the angular displacement can be controlled stepwise in the auto-focusing system for a photographing lens. In a 1-2 phase excitation system stepping motor (half-step drive system) which has a large rotational torque and a simple control circuit, if the power supply is stopped at a 1-phase excitation position, i.e., if no excitation occurs, the rotor is stopped at the 1-phase excitation position (detent position) . However, if the power supply is stopped at a 2-phase excitation position, the rotor is in an equilibrium state between the detent positions and hence, the rotor tends to rotate to one of the detent positions. Consequently, if the excitation is ceased at the 2-phase excitation position, the rotor rotates to and stops at a detent position adjacent to the 2-phase excitation position (equilibrium position, stable position), thus resulting in an occurrence of an error in the stop position. In order to prevent this, it is necessary to hold the 2-phase excitation to maintain the equilibrium position when the power supply is ceased at the 2-phase excitation position. This invites an increase in the power consumption.
Furthermore, in recent digital cameras or digital still cameras, a CCD image pickup device has been used in place of a silver-halide film. In general, the latitude of the CCD image pickup device is narrower than that of the silver-halide film, and hence it is necessary to precisely adjust the quantity of exposure light or the focus upon photographing. However, if, for example, a photographer views an object through an optical finder or liquid crystal display, when no photographing operation is carried out, the latitude is such that the user cannot be aware of any blurriness.
It is an object of the present invention to provide a drive apparatus for a stepping motor of a camera, in which a precise focus control can be effected and the power consumption of a 1-2 phase excitation stepping motor can be reduced.